Does fuel economy get better with altitude?

[bombloader]

Thanks guys. I actually understand pretty well how it affects aircraft. I know in theory it should have some effect on a car, but I also know that its really hard to test because of the aforementioned problems such as more hills in high elevation places. Just wondering if the difference was every noticeable, or if it was small enough that it got swamped in other effects. Sounds like at least some of you have noticed though. Gonna take everyone's word for it for now, because I don't feel like A-B-A testing the whole thing by making a few runs in Denver BTW, why the suggestions that some diesels would be different? Only difference I can see is that most of them are turbocharged, so you wouldn't lose power at altitude. Still, less drag=less power required to maintain a given speed.

[almightybmw]

I'll throw my two cents in since I live at about 5000ft: on the roads here I can average 34mpg in my GP, up and down the passes. Take a look at my gas log. I don't update it anymore, haven't the time really. But you'll see that between here and Coure De Alene I can get 34mpg with a fully loaded car, up a few thousand feet and down. When I first bought the car and drove it back from OR it managed 34mpg between the west side of Portland to Coure De Alene, so through big city traffic. I wasn't doing any hypermiling tricks, just set the cruise at 70mph and go. Come to think of it, I drove it to CA just after buying it, and averaged 32mpg over those 2000 miles, with lots of go pedal.

So my experience isn't that great of a definition. I can get the same at sea level to 6500ft. Granted those two experiences were over 2 years apart, but hey. I think the only corrolation I find is at lower elevations the car really shows its full power, which is quite fun. Its almost anemic up here, so I tend to "enjoy" it more down there, sporting with traffic and all. Plus there's the whole how much throttle needed, amount of air being sucked in, blah blah the normal stuff.

Bottom line, yes, the car gets better mileage at higher altitudes, BUT (big big but here) it's such a gas to drive down there and so boring up here (even more so when I was in the high lands of CO), the amount of power available for a given throttle input, equates to crap mileage down low and average to good mileage up high.

hmm. long post. And all I said was I love using the go pedal. hmm. At least I keep it above the 30mpg floor!

[wagonman76]

I didn't have a mpg gauge at the time, but when I towed my camper through the Porcupine mountiains in the UP, I noticed it was a lot easier to maintain 55 mph the further up I got, plus I was going uphill. A lot easier than any of the flat roads at lower elevation. Only reason I could think of was the air was thinner and less wind resistance.

[ConnClark]

Not only does power drop with altitude but engine efficiency does as well. It all comes down to whether the drop in aerodynamic drag is greater than the drop in engine efficiency. I would tend to doubt this is the case though.

[aerohead]

Friend and workmate Steve relayed an account of Mike Sizer's Honda Civic ( late model ) which was returning 36-38 mpg HWY,then on trip to Yellowstone National Park and back,achieved up to 44-mpg in the higher elevations.Mikes no slacker ( professional musician and Air Traffic Controller @ Ft.Worth Center ) and I would not discount his claims.He accounts thinner air and ability of electronic fuel injection to compensate,for improvement.

[winkosmosis]

Originally Posted by ConnClark

Not only does power drop with altitude but engine efficiency does as well. It all comes down to whether the drop in aerodynamic drag is greater than the drop in engine efficiency. I would tend to doubt this is the case though.

I don't think there is a drop in efficiency. Why would people run hot air intakes if thin air didn't mean more efficiency?

[winkosmosis]

Originally Posted by aerohead

Friend and workmate Steve relayed an account of Mike Sizer's Honda Civic ( late model ) which was returning 36-38 mpg HWY,then on trip to Yellowstone National Park and back,achieved up to 44-mpg in the higher elevations.Mikes no slacker ( professional musician and Air Traffic Controller @ Ft.Worth Center ) and I would not discount his claims.He accounts thinner air and ability of electronic fuel injection to compensate,for improvement.

Coincidentally Yellowstone is where I got good gas mileage with the Grand Cherokee. I worked there as a GIS intern in 2006. Even with all the hills, I got better gas mileage than I got in Houston.

[aerohead]

Originally Posted by winkosmosis

Coincidentally Yellowstone is where I got good gas mileage with the Grand Cherokee. I worked there as a GIS intern in 2006. Even with all the hills, I got better gas mileage than I got in Houston.

Looks like all the gee-whiz electronic gizmos are really a good friend and ally.

[winkosmosis]

Originally Posted by aerohead

Looks like all the gee-whiz electronic gizmos are really a good friend and ally.

Yeah they are. With an instant mpg readout, you see how much your mileage drops going uphill, and you reduce throttle accordingly.

[bombloader]

Originally Posted by ConnClark

Not only does power drop with altitude but engine efficiency does as well. It all comes down to whether the drop in aerodynamic drag is greater than the drop in engine efficiency. I would tend to doubt this is the case though.

Are you referring to the case of carbureted engines? Most non-aviation carbureted engine have fixed jetting for mixture, so yeah efficiency does decrease because the mixture becomes richer. You have to rejet the whole carburetor to fix the problem. Modern injected engines maintain a stoichometric mixture constantly, so that wouldn't be a problem.